Data networks have become increasingly complex and it is a tremendous task to maintain and to monitor a policy that determines the authorizations for accessing private information. In the state of the art it is well known to use firewalls, for example to limit the access to a private network that is connected to the Internet only to users having the allowance and the key to access the private network. It is known from the article “The design of grids: a graph-based intrusion detection system” by Stephen Chung et al., Department of Computer Science, University of California at Davis, Jan. 26, 1999, to use computer-graphs for detecting an attack from outside to a private network. In that system, unauthorized accesses from outside were detected as graphs and compared to given graphs designed according to a given network policy. If an access from outside results in a graph that does not fit with the given graph, then an attack is detected. Intrusions which are small, slow, or both might not be detected by that system. Hence, it is still possible for intruders to intrude into the system.
The known systems have the disadvantage that a failure in protecting confidential information in a network is detected after the attack. This means that in most cases the confidential information may be transferred to the attacker before the unauthorized access has been detected.